Method and Apparatus of Luma-Chroma Separated Coding Tree Coding with Constraints

ABSTRACT

A method and apparatus for block partition are disclosed. If a cross-colour component prediction mode is allowed, the luma block and the chroma block are partitioned into one or more luma leaf blocks and chroma leaf blocks. If a cross-colour component prediction mode is allowed, whether to enable an LM (Linear Model) mode for a target chroma leaf block is determined based on a first split type applied to an ancestor chroma node of the target chroma leaf block and a second split type applied to a corresponding ancestor luma node. According to another method, after the luma block and the chroma block are partitioned using different partition tress, determine whether one or more exception conditions to allow an LM for a target chroma leaf block are satisfied when the chroma partition tree uses a different split type, a different partition direction, or both from the luma partition tree.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority to U.S. Provisional Patent Application, Ser. No. 62/786,530, filed on Dec. 30, 2018, and U.S. Provisional Patent Application, Ser. No. 62/865,397, filed on Jun. 24, 2019. The U.S. Provisional Patent Applications are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to block partition of luma and chroma blocks using separate partition trees in video coding. In particular, the present invention discloses control schemes of dependent cross component coding to reduce complexity or required system resources.

BACKGROUND AND RELATED ART

The High Efficiency Video Coding (HEVC) standard is developed under the joint video project of the ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG) standardization organizations, and is especially with partnership known as the Joint Collaborative Team on Video Coding (JCT-VC). In HEVC, one slice is partitioned into multiple coding tree units (CTU). In main profile, the minimum and the maximum sizes of CTU are specified by the syntax elements in the sequence parameter set (SPS). The allowed CTU size can be 8×8, 16×16, 32×32, or 64×64. For each slice, the CTUs within the slice are processed according to a raster scan order.

The CTU is further partitioned into multiple coding units (CU) to adapt to various local characteristics. A quadtree, denoted as the coding tree, is used to partition the CTU into multiple CUs. Let CTU size be M×M, where M is one of the values of 64, 32, or 16. The CTU can be a single CU (i.e., no splitting) or can be split into four smaller units of equal sizes (i.e., M/2×M/2 each), which correspond to the nodes of the coding tree. If units are leaf nodes of the coding tree, the units become CUs. Otherwise, the quadtree splitting process can be iterated until the size for a node reaches a minimum allowed CU size as specified in the SPS (Sequence Parameter Set). This representation results in a recursive structure as specified by a coding tree (also referred to as a partition tree structure) 120 in FIG. 1. The CTU partition 110 is shown in FIG. 1, where the solid lines indicate CU boundaries. The decision whether to code a picture area using Inter-picture (temporal) or Intra-picture (spatial) prediction is made at the CU level. Since the minimum CU size can be 8×8, the minimum granularity for switching between different basic prediction types is 8×8.

Furthermore, according to HEVC, each CU can be partitioned into one or more prediction units (PU). Coupled with the CU, the PU works as a basic representative block for sharing the prediction information. Inside each PU, the same prediction process is applied and the relevant information is transmitted to the decoder on a PU basis. A CU can be split into one, two or four PUs according to the PU splitting type. HEVC defines eight shapes for splitting a CU into PU as shown in FIG. 2, including 2N×2N, 2N×N, N×2N, N×N, 2N×nU, 2N×nD, nL×2N and nR×2N partition types. Unlike the CU, the PU may only be split once according to HEVC. The partitions shown in the second row correspond to asymmetric partitions, where the two partitioned parts have different sizes.

After obtaining the residual block by the prediction process based on PU splitting type, the prediction residues of a CU can be partitioned into transform units (TU) according to another quadtree structure which is analogous to the coding tree for the CU as shown in FIG. 1. The solid lines indicate CU boundaries and dotted lines indicate TU boundaries. The TU is a basic representative block having residual or transform coefficients for applying the integer transform and quantization. For each TU, one integer transform having the same size to the TU is applied to obtain residual coefficients. These coefficients are transmitted to the decoder after quantization on a TU basis.

The terms coding tree block (CTB), coding block (CB), prediction block (PB), and transform block (TB) are defined to specify the 2-D sample array of one colour component associated with CTU, CU, PU, and TU, respectively. Thus, a CTU consists of one luma CTB, two chroma CTBs, and associated syntax elements. A similar relationship is valid for CU, PU, and TU. The tree partitioning is generally applied simultaneously to both luma and chroma, although exceptions apply when certain minimum sizes are reached for chroma.

Alternatively, a binary tree block partitioning structure is proposed in JCTVC-P1005 (D. Flynn, et al, “HEVC Range Extensions Draft 6”, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 16th Meeting: San Jose, U.S., 9-17 Jan. 2014, Document: JCTVC-P1005). In the proposed binary tree partitioning structure, a block can be recursively split into two smaller blocks using various binary splitting types as shown in FIG. 3. The most efficient and simplest ones are the symmetric horizontal and vertical split as shown in the top two splitting types in FIG. 3. For a given block of size M×N, a flag is signalled to indicate whether the given block is split into two smaller blocks. If yes, another syntax element is signalled to indicate which splitting type is used. If the horizontal splitting is used, the given block is split into two blocks of size M×N/2. If the vertical splitting is used, the given block is split into two blocks of size M/2×N. The binary tree splitting process can be iterated until the size (width or height) for a splitting block reaches a minimum allowed block size (width or height). The minimum allowed block size can be defined in high level syntax such as SPS. Since the binary tree has two splitting types (i.e., horizontal and vertical), the minimum allowed block width and height should be both indicated. Non-horizontal splitting is implicitly implied when splitting would result in a block height smaller than the indicated minimum. Non-vertical splitting is implicitly implied when splitting would result in a block width smaller than the indicated minimum. FIG. 4 illustrates an example of block partitioning 410 and its corresponding binary tree 420. In each splitting node (i.e., non-leaf node) of the binary tree, one flag is used to indicate which splitting type (horizontal or vertical) is used, where 0 may indicate horizontal splitting and 1 may indicate vertical splitting.

The binary tree structure can be used for partitioning an image area into multiple smaller blocks such as partitioning a slice into CTUs, a CTU into CUs, a CU into PUs, or a CU into TUs, and so on. The binary tree can be used for partitioning a CTU into CUs, where the root node of the binary tree is a CTU and the leaf node of the binary tree is CU. The leaf nodes can be further processed by prediction and transform coding. For simplification, there is no further partitioning from CU to PU or from CU to TU, which means CU equal to PU and PU equal to TU. Therefore, in other words, the leaf node of the binary tree is the basic unit for prediction and transforms coding.

QTBT Structure

Binary tree structure is more flexible than quadtree structure since more partition shapes can be supported, which is also the source of coding efficiency improvement. However, the encoding complexity will also increase in order to select the best partition shape. In order to balance the complexity and coding efficiency, a method to combine the quadtree and binary tree structure, which is also called as quadtree plus binary tree (QTBT) structure, has been disclosed. According to the QTBT structure, a CTU (or CTB for I slice) is the root node of a quadtree and the CTU is firstly partitioned by a quadtree, where the quadtree splitting of one node can be iterated until the node reaches the minimum allowed quadtree leaf node size (i.e., MinQTSize). If the quadtree leaf node size is not larger than the maximum allowed binary tree root node size (i.e., MaxBTSize), it can be further partitioned by a binary tree. The binary tree splitting of one node can be iterated until the node reaches the minimum allowed binary tree leaf node size (i.e., MinBT Size) or the maximum allowed binary tree depth (i.e., MaxBTDepth). The binary tree leaf node, namely CU (or CB for I slice), will be used for prediction (e.g. Intra-picture or inter-picture prediction) and transform without any further partitioning. There are two splitting types in the binary tree splitting: symmetric horizontal splitting and symmetric vertical splitting. In the QTBT structure, the minimum allowed quadtree leaf node size, the maximum allowed binary tree root node size, the minimum allowed binary tree leaf node width and height, and the maximum allowed binary tree depth can be indicated in the high level syntax such as in SPS. FIG. 5 illustrates an example of block partitioning 510 and its corresponding QTBT 520. The solid lines indicate quadtree splitting and dotted lines indicate binary tree splitting. In each splitting node (i.e., non-leaf node) of the binary tree, one flag indicates which splitting type (horizontal or vertical) is used, 0 may indicate horizontal splitting and 1 may indicate vertical splitting.

The above QTBT structure can be used for partitioning an image area (e.g. a slice, CTU or CU) into multiple smaller blocks such as partitioning a slice into CTUs, a CTU into CUs, a CU into PUs, or a CU into TUs, and so on. For example, the QTBT can be used for partitioning a CTU into CUs, where the root node of the QTBT is a CTU which is partitioned into multiple CUs by a QTBT structure and the CUs are further processed by prediction and transform coding. For simplification, there is no further partitioning from CU to PU or from CU to TU. That means CU equal to PU and PU equal to TU. Therefore, in other words, the leaf node of the QTBT structure is the basic unit for prediction and transform.

An example of QTBT structure is shown as follows. For a CTU with size 128×128, the minimum allowed quadtree leaf node size is set to 16×16, the maximum allowed binary tree root node size is set to 64×64, the minimum allowed binary tree leaf node width and height both is set to 4, and the maximum allowed binary tree depth is set to 4. Firstly, the CTU is partitioned by a quadtree structure and the leaf quadtree unit may have size from 16×16 (i.e., minimum allowed quadtree leaf node size) to 128×128 (equal to CTU size, i.e., no split). If the leaf quadtree unit is 128×128, it cannot be further split by binary tree since the size exceeds the maximum allowed binary tree root node size 64×64. Otherwise, the leaf quadtree unit can be further split by binary tree. The leaf quadtree unit, which is also the root binary tree unit, has binary tree depth as 0. When the binary tree depth reaches 4 (i.e., the maximum allowed binary tree as indicated), no splitting is implicitly implied. When the block of a corresponding binary tree node has width equal to 4, non-horizontal splitting is implicitly implied. When the block of a corresponding binary tree node has height equal to 4, non-vertical splitting is implicitly implied. The leaf nodes of the QTBT are further processed by prediction (Intra picture or Inter picture) and transform coding.

For I-slice, the QTBT tree structure usually applied with the luma/chroma separate coding. For example, the QTBT tree structure is applied separately to luma and chroma components for I-slice, and applied simultaneously to both luma and chroma (except when certain minimum sizes being reached for chroma) for P- and B-slices. In other words, in an I-slice, the luma CTB has its QTBT-structured block partitioning and the two chroma CTBs have another QTBT-structured block partitioning. In another example, the two chroma CTBs can also have their own QTBT-structured block partitions.

LM Chroma Mode

The Intra predictor is usually designed to exploit spatial features in the picture such as smooth area (DC mode), vertical line or edge, horizontal line or edge and diagonal line or edge. Furthermore, spatial correlation often exists between the luminance (luma) and chrominance (chroma) components. Therefore, reconstructed luma pixels can be used to derive the Intra chroma prediction. In the emerging High Efficiency Video Coding (HEVC), a chroma Intra prediction mode based on the reconstructed luminance signal has been considered. This type of chroma Intra prediction is termed as Linear Model (LM) prediction. FIG. 6 illustrates the Intra prediction derivation for LM mode. First, the neighbouring reconstructed pixels (indicated by circles) of a collocated luma block (i.e., Y block) and the neighbouring reconstructed pixels (indicated by circles) of a chroma block (i.e., U or V block) in FIG. 6 are used to derive the linear model parameters between the blocks. The predicted pixels of the chroma block are generated using the parameters and the reconstructed pixels of the luma block. In the parameters derivation, the top reconstructed pixel row adjacent to the top block boundary of the current luma block and the left reconstructed pixel column adjacent to the left block boundary of the current luma block are used. It is noted that the second left reconstructed pixel column from the left boundary is used instead of the left column immediately adjacent to the left boundary in order to match the sampling locations of the chroma pixels. The specific row and column of the luma block are used in order to match the 4:2:0 sampling format of the chroma components. While FIG. 6 illustrates the example of LM chroma mode for the 4:2:0 sampling format, the LM chroma mode for other chroma sampling format may also derived similarly.

According to the LM prediction mode, the chroma values are predicted from reconstructed luma values of a collocated block. The chroma components may have lower spatial resolution than the luma component. In order to use the luma signal for chroma Intra prediction, the resolution of the luma signal may have to be reduced to match with that of the chroma components. For example, for the 4:2:0 sampling format, the U and V components only have half of the number of samples in vertical and horizontal directions as the luma component. Therefore, 2:1 resolution reduction in vertical and horizontal directions has to be applied to the reconstructed luma samples. The resolution reduction can be achieved by down-sampling process or sub-sampling process.

In LM chroma mode, for a to-be-predicted chroma sample V with its collocated reconstructed luma sample V_(col), the linear model to generate LM predictor P is formulated as follows:

P=a·V _(col) +b,

In the above equation, a and b are referred as LM parameters. The LM parameters can be derived from the neighbouring reconstructed luma and chroma samples around the current block so that the parameters do not need to be coded in the bitstream. After deriving the LM parameters, chroma predictors can be generated from the collocated reconstructed luma samples in the current block according to the linear model. For example, if the video format is YUV420, then there are one 8×8 luma block (610) and two 4×4 chroma blocks (620 and 630) for each 8×8 coding unit, as shown in FIG. 6. In FIG. 6, each small square corresponds to one pixel in the current coding unit (2N×2N for luma and N×N for chroma) to be coded. The LM parameters are derived first based on neighbouring reconstructed samples of the current coding unit, which are represented as circles in FIG. 6. Due to the YUV420 sampling format, the collocated chroma position is located between two corresponding vertical luma samples. An average value between two corresponding vertical luma samples is used to derive the LM parameters. For neighbouring pixels above the top block boundary, the average value is replaced by the closest sample in the vertical direction in order to reduce the line buffer requirement. The neighbouring pixels (as shown in circles) of the currently luma (Y) and chroma (U or V) coding units are used to derive the LM parameters for the respective chroma component as shown in FIG. 6. After the LM parameters are derived, the chroma predictors are generated based on the linear model and the collocated luma reconstructed samples. According to the video format, an average luma value may be used instead of the corresponding luma sample.

Since the tree structure of luma and chroma components are separated and the data dependency between luma and chroma components exists, the chroma component cannot be processed until the process of the corresponding luma component is finished. For example, in LM chroma mode, the chroma predictors depend on the luma reconstructed samples. Only when the luma reconstruction process is finished and the luma reconstructed samples are ready, then the chroma predictors can be generated. The latency in the worst case will become one CTU or one unit with interleaved luma and chroma components. For example, if the tree structures of luma and chroma components are interleaved organized in one 64×64 unit by one 64×64 unit, then the chroma component cannot be processed until the process of the corresponding luma 64×64 block is finished. This introduces the buffer usage in a pipeline video coding system. The present invention discloses various methods to reduce the buffer requirement as well latency issue.

BRIEF SUMMARY OF THE INVENTION

A method and apparatus for block partition in video encoding and decoding are disclosed. According to one method of the present invention, input data associated with a current block in a current picture are received, where the input data comprise pixel data to be encoded at an encoder side or compressed data at a decoder side, and wherein the current block comprises a luma block and a chroma block. If a cross-colour component prediction mode is allowed, the luma block is partitioned into one or more luma leaf blocks using a luma partition tree and the chroma block is partitioned into one or more chroma leaf blocks using a chroma partition tree. If a cross-colour component prediction mode is allowed, whether to enable an LM (Linear Model) mode for a target chroma leaf block is determined based on a first split type applied to an ancestor chroma node of the target chroma leaf block and a second split type applied to a corresponding ancestor luma node. The target chroma leaf block is encoded or decoded using the LM mode if the LM mode is enabled for the target chroma leaf block.

In one embodiment, the corresponding ancestor luma node has a block size of 64×64 and the ancestor chroma node of the target chroma leaf block has a block size of 32×32 when the current picture is in a 4:2:0 colour format. When the second split type corresponds to no split and the first split type corresponds to no split, QT (quadtree) split, HorBT (horizontal binary tree) split with no further split, or HorBT split followed by VerBT (vertical binary tree) split, the LM mode for the target chroma leaf block may be enabled. When the second split type corresponds to QT (quadtree) split and the first split type corresponds to no split, QT (quadtree) split, HorBT (horizontal binary tree) split with no further split, or HorBT split followed by VerBT (vertical binary tree) split, the LM mode for the target chroma leaf block may be enabled.

Another method and apparatus for block partition in video encoding and decoding are also disclosed. After the luma block and the chroma block are partitioned using different partition tress, determine whether one or more exception conditions to allow an LM (Linear Mode) for a target chroma leaf block are satisfied when the chroma partition tree uses a different split type, a different partition direction of both from the luma partition tree. The target chroma leaf block is encoded or decoded using the LM mode if the LM mode is enabled for the target chroma leaf block.

According to one embodiment, said one or more exception conditions comprise a case that the luma block being a luma 64×64 block is partitioned into four luma 32×32 blocks using a QT (quadtree) split and the chroma block being a chroma 32×32 block is partitioned into two chroma 32×16 blocks using a horizontal BT (binary tree) split with no further split or partitioned into four chroma 16×16 blocks using the horizontal BT split followed by a vertical BT split. According to another embodiment, said one or more exception conditions comprise a case that the luma block being a luma 64×64 block is partitioned into four luma 32×32 blocks using a horizontal BT (binary tree) split followed by a vertical BT split and the chroma block being a chroma 32×32 block is partitioned into four chroma 16×16 blocks using a QT (quadtree) split. In still yet another embodiment, said one or more exception conditions comprise a case that the chroma block is partitioned into one or more split chroma CUs (coding units) and each of said one or more split chroma CUs covers and contains one or more complete luma CUs resulted from partitioning the luma block using the luma partition tree, and wherein coding order of said one or more complete luma CUs are continuous.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of block partition using quadtree structure to partition a coding tree unit (CTU) into coding units (CUs).

FIG. 2 illustrates asymmetric motion partition (AMP) according to High Efficiency Video Coding (HEVC), where the AMP defines eight shapes for splitting a CU into PU.

FIG. 3 illustrates an example of various binary splitting types used by a binary tree partitioning structure, where a block can be recursively split into two smaller blocks using the splitting types.

FIG. 4 illustrates an example of block partitioning and its corresponding binary tree, where in each splitting node (i.e., non-leaf node) of the binary tree, one syntax is used to indicate which splitting type (horizontal or vertical) is used, where 0 may indicate horizontal splitting and 1 may indicate vertical splitting.

FIG. 5 illustrates an example of block partitioning and its corresponding quad-tree plus binary tree structure (QTBT), where the solid lines indicate quadtree splitting and dotted lines indicate binary tree splitting.

FIG. 6 illustrates an example of chroma Intra prediction (i.e., Linear Model (LM) prediction), where the neighbouring reconstructed pixels (indicated by circles) of a collocated luma block (i.e., Y block) and the neighbouring reconstructed pixels (indicated by circles) of a chroma block (i.e., U or V block) are used to derive the linear model parameters between the blocks.

FIG. 7 illustrates an example of the constrained Luma-Chroma separated coding tree coding, where the left block is the tree structure of luma component and the right block is the tree structure of chroma component.

FIG. 8 illustrates examples of implicit TU split for 32×32 chroma block according to 16×16 pipeline processing grids, where if 32×32 chroma block is not further split, then the implicit TU split is not applied.

FIG. 9 illustrates examples for checking if the neighbouring reconstructed reference samples are available for blocks “A” and “B”.

FIG. 10 illustrates examples of implicit TU split for 32×32 chroma block according to 8×8 pipeline processing grids, where if 32×32 chroma block is not further split, then the implicit TU split is not applied.

FIG. 11 illustrates examples of implicit TU split for 32×32 chroma block according to 16×16 pipeline processing grids if only no split/QT split is applied to 64×64 luma modes, where if 32×32 or 32×16 chroma block is not further split, then the implicit TU split is not applied.

FIG. 12 illustrates a flowchart of an exemplary coding system according to an embodiment of the present invention, where whether to allow the LM mode depends on splitting at the parent nodes.

FIG. 13 illustrates a flowchart of an exemplary coding system according to an embodiment of the present invention, where exception conditions are determined to allow the LM mode when the luma and chroma use different splitting.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

The latency is introduced by the data dependency between different tree structures. In order to reduce the latency, an invention is disclosed to constrain the data dependency between different tree structures. The idea is to allow the data dependency between different tree structures only when the tree structures are mutually inclusive. In other words, when the splitting is not used in one tree or when the splitting is used in one tree and the same splitting or no split is used in the other tree, the data dependency between different tree structures is allowed. When the splitting in different tree structures are different, then the data dependency between different tree structures is disallowed. The “splitting” here refers to splitting at one node in one tree structure or another node in another tree structure. The “splitting one tree” and “the . . . splitting . . . in the other tree” refer to “splitting one node in one tree structure” and “splitting another node in another tree structure”, where “one node in one tree structure” and “another node in another tree structure” are two corresponding nodes in two tree structures.

In other words, when the data dependency between different tree structures is allowed, each leaf CU in the chroma tree should include one or more complete luma leaf CUs or it is completely included in one luma leaf CU. Otherwise, the data dependency between different tree structures is disallowed. In one embodiment, the data dependency between different tree structures is LM chroma mode and/or RM mode. In other words, if the data dependency between different tree structures is allowed, at least one mode with the data dependency between different tree structures (for example, LM chroma mode and/or RM mode) can be a candidate mode being applied; otherwise (i.e., the data dependency between different tree structures is disallowed), such mode with the data dependency between different tree structures (for example, LM chroma mode and/or RM mode) cannot be applied. The RM mode corresponds to residual prediction, such as predicting chroma residual data from luma residual data.

FIG. 7 illustrates an example of the constrained Luma-Chroma separated coding tree coding, where the left block 710 is the tree structure of luma component and the right block 720 is the tree structure of chroma component. For one chroma leaf CU (i.e., a, b, m, n, or o) that cannot be completely included in one luma leaf CU or doesn't include one or more complete luma leaf CUs, the data dependency between different tree structures (i.e., LM chroma mode) is disallowed. For one chroma leaf CU (i.e., c or i) which includes one or more complete luma leaf CUs, the data dependency between different tree structures (i.e., LM chroma mode) is allowed. For one chroma leaf CU (i.e., d, e, f, g, h, j, or k), which is completely included in one luma leaf CU, the data dependency between different tree structures (i.e., LM chroma mode) is also allowed. By using this constraint, the latency of processing chroma Intra mode can be reduced. For example, for chroma leaf CU c, when luma leaf CU D is reconstructed, the top-left block in chroma leaf CU c can be processed before processing luma leaf CU I. In some embodiments, since the data dependency exists between chroma leaf CU a and luma leaf CUB, the reconstruction process of chroma leaf CU a must be processed independently without considering the reconstruction process of luma leaf CUs A and B.

FIG. 7 corresponds to the resulting luma leaf CUs and chroma leaf CUs from respective partition trees, where the whole block corresponds to a root node in a respective partition tree. Each leaf CU corresponds to one leaf node in the partition tree. As shown in FIG. 7, the first-level partition splits the root node into four child nodes. The partitioning for the root node can be a quadtree (QT) split. However, the partitioning can also correspond to horizontal binary (H. BT) partition followed by vertical binary partition (V. BT) or vertical binary partition (V. BT) followed by horizontal binary (H. BT) partition. When two partitioning structures result in the same partitions, the two partitioning structures are referred as equivalent partition or equivalent splitting in this disclosure. In FIG. 7, leaf CU c (722) of the chroma tree 720 covers the same area 712 consisting of leaf CUs D, E, F, G, H and I of the luma tree 710. The chroma node in the chroma tree associated with leaf CU c corresponds to a luma node in the luma tree associated with leaf CUs, D, E, F, G, H and I. In this example, the chroma node associated with leaf CU c is not split. On the other hand, the luma node in the luma tree associated with leaf CUs D, E, F, G, H and I is split by a splitting structure to result in leaf CUs D, E, F, G, H and I. The chroma node associated with leaf CU c and the luma node in the luma tree associated with leaf CUs D, E, F, G, H and I are referred as corresponding nodes in two trees in this disclosure.

The data dependency between different tree structures refers to the data dependency between one or more leaf CUs resulted from one node in one tree and one or more leaf CUs resulted from a corresponding node in another tree. For example, data dependency can be applied between chroma leaf CU c and luma leaf CUs D, E, F, G, H and I.

In another embodiment, for each leaf CU in chroma tree, only when it includes one or more complete luma leaf CUs, the data dependency between different tree structures is allowed. Otherwise, the data dependency between different tree structures is disallowed.

In another embodiment, for each leaf CU in the chroma tree, only when it is completely included in one luma leaf CU, the data dependency between different tree structures is allowed. Otherwise, the data dependency between different tree structures is disallowed.

In another embodiment, for each leaf CU in the chroma tree, only when it is completely included in one luma leaf CU and the corresponding luma leaf CU includes one or more complete leaf chroma CUs, the data dependency between different tree structures is allowed. Otherwise, the data dependency between different tree structures is disallowed.

In another embodiment, the data dependency between different tree structures is allowed when the splitting is not used in one tree or when the splitting is used in one tree and the same splitting or no splitting is used in another tree.

In another embodiment, the data dependency between different tree structures is allowed only when the same splitting structures are used in different trees. In one embodiment, one flag is signalled or inferred at the CTU level or block level to indicate whether the same splitting structures are used in different trees or not. If the same splitting structures are used in different trees, then one of the tree structures can be saved and not signalled. The block level can be defined as the maximum transform size or some pre-defined value according to the profile or the level in one video coding standard. According to another embodiment, the value can be signalled at the sequence level, picture level, slice level, tile group level or tile level.

In another embodiment, the data dependency between different tree structures is allowed when the splitting is used in one tree and the same splitting or no splitting is used in another tree.

In another embodiment, when the different splitting partitions are used for different tree structures, the data dependency between different tree structures is disallowed for those leaf CUs generated by different splitting partitions.

In one embodiment, one or more of the above embodiments can be combined to allow the data dependency between different tree structures while the data dependency between different tree structures may be disallowed other than such combination under different conditions.

In one embodiment, when the data dependency between different tree structures is disallowed, the syntax which indicates the data dependency between different tree structures is used or not is forced to be a predefined value corresponding to not to use the data dependency at the encoder, where the decoder may apply such feature correspondingly (for example, when the data dependency between different tree structures is disallowed, the syntax which indicates the data dependency between different tree structures is used or not is forced to be a predefined value corresponding to not to use the data dependency at the decoder).

In another embodiment, when the data dependency between different tree structures is disallowed, the syntax which indicates the data dependency between different tree structures is used or not is inferred to be one predefined value corresponding to not to use the data dependency, at the encoder, where the decoder may apply such feature correspondingly (for example, when the data dependency between different tree structures is disallowed, the syntax which indicates the data dependency between different tree structures is used or not is inferred to be one predefined value corresponding to not to use the data dependency at the decoder).

In another embodiment, when the data dependency between different tree structures is disallowed, the mode with the data dependency between different tree structures is replaced by a predefined mode without the data dependency.

In the original design of separate partitioning for luma and chroma components, the shapes of luma and chroma blocks can be different, and hence a luma block can extend across different chroma blocks and vice versa. One consequence is that luma and chroma blocks must be processed separately, and the buffered samples for prediction and reconstruction may need to be kept in memory until the entire unit has been processed. On the other hand, if, for example, none of the luma blocks extends across different chroma blocks (i.e., each luma block being entirely inside a chroma block), then after processing a specific chroma block and the luma blocks within, the buffered samples for processing this block can be discarded. To achieve this, we can apply the same partitioning to both luma and chroma blocks multiple times, and then disable further splitting for one of the components. Whether to disable further splitting can be explicitly signalled or implicitly derived based on the size or depth of the current block.

In one embodiment, a flag called chroma_split_end is signalled before or after each luma split flags or syntaxes, indicating whether chroma partitioning terminates at this point, and the subsequent splits will be applied only to the luma component. Before the chroma_split_end is true, the chroma partition is the same as the luma partition. The chroma_split_end flag can be used to indicate that the corresponding split flag represents the last partitioning applied to the chroma component, or that the corresponding split flag and subsequent split flags are not applicable to the chroma component. In one example, when reaching the luma leaf CU and the chroma partition is not terminated (e.g. the chroma_split_end being all false), the chroma_split_end is inferred as true, which means the chroma component is not split anymore. In another example, when reaching a luma leaf CU and the chroma partition is not terminated, the chroma_split syntax or CU split syntaxes for chroma component are applied. The chroma component can be further partitioned into smaller CUs. In one example, when the luma partition is not split, a chroma_split_end is signalled. If the flag is true, the QT/BT/TT split syntaxes are signalled. In another example, when the luma partition is not split, the chroma CU split syntaxes are signalled. If the no split is selected, the chroma CU also stops split. Otherwise, the chroma CU can be further split. The chroma component split syntaxes can follow the rule of CU split. For example, when the binary tree (BT) or ternary tree (TT) split is used, the QT split is not allowed. The chroma QT/BT/TT size/shape/depth constraints can also be applied. For example, if the maximum chroma QT/BT/TT depth or minimum chroma QT/BT/TT size/shape is reached, the chroma_split_end is inferred as false and not signalled. The chroma_split_end can be signalled or inferred when the luma component is coding quad-tree split, binary-tree split, or ternary-tree split. In one example, the chroma_split_end can be signalled or inferred when luma component is coding quad-tree split. If the QT split is before the BT and/or TT split, it means when luma component is doing QT-split, the chroma component needs to follow the QT split or stop the split. When luma component is doing BT/TT-split, the chroma is free to using BT/TT split. In another example, the chroma QT-split can be stopped before the luma QT leaf CU, and the chroma QT leaf CU can be further split by using BT/TT split.

In another embodiment, a flag called chroma_follow_luma_split_flag is used to determine whether the chroma follows the luma split. If the flag is true, the luma and chroma use the same split partition. When a luma CU uses QT/BT/TT split, the chroma CU also uses QT/BT/TT split. When the luma CU stops split, the chroma CU also stops split. If the flag is false, the luma and chroma use different split decision. When luma CU uses QT/BT/TT split, the chroma CU stops split. When the luma CU stops split, the chroma CU is further split. The chroma split syntaxes is further signalled. In one example, when the luma CU stops split, the chroma_follow_luma_split_flag is inferred as true. In another example, when the maximum chroma QT/BT/TT depth or minimum chroma QT/BT/TT size/shape is reached, the chroma_follow_luma_split_flag can also be inferred.

In another embodiment, a flag called luma_split_end is signalled before or after each split flag and the subsequent splits will be applied only to the chroma component. The luma_split_end flag indicates whether luma partitioning terminates at this point. The luma_split_end flag can be used to indicate that the corresponding split flag represents the last partitioning applied to the luma component, or that the corresponding split flag and subsequent split flags are not applicable to the luma component. In one example, when the chroma leaf CU is reached and the luma partition is not terminated (e.g. the luma_split_end are all false), the luma_split_end is inferred as true, which means the luma component is not split anymore. In another example, when the chroma leaf CU is reached and the luma partition is not terminated, the luma split syntax is applied. The luma component can be further partitioned into smaller CUs.

In one embodiment, the CTU is split or inferred split (e.g. inferred QT-split) to a predefined size/shape. The predefined size/shape can be the same as the maximum transform unit size, an M×N block, or an area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64), a derived block size, maximum TU size, a size related to maximum/minimum TU size, or an adaptive shape block with area equal to a predefined or a derived block area (e.g. block area equal to 4096, or the virtual pipeline data unit (VPDU) area/size). After split to the predefined size/shape, the proposed CU split method is applied. In one example, the proposed method is only applied in Intra-slice or Intra-tile or Intra-tile group or when dual-tree is enabled.

In another embodiment, when CU size is larger than a predefined size/shape, the share tree partitioning method is used. When the CU is not split and the size is larger than the predefined size/shape, both luma and chroma component are not split. The chroma_split_end syntax is not signalled. The predefined size/shape can be the same as the maximum transform unit size, an M×N block, or an area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64), a derived block size, maximum TU size, or a size related to maximum/minimum TU size, or an adaptive shape block with area equal to a predefined or a derived block area (e.g. block area equal to 4096 or the virtual pipeline data unit (VPDU) area/size). When the CU size is smaller or equal to the predefined size/shape, the proposed method is applied. For example, the chroma_split_end or chroma CU split syntaxes can be signalled.

In another embodiment, the proposed checking if chroma partition follows luma partition, which can be, for example, used for determining whether LM mode is enabled, is only applied when the chroma partition size is larger than (or larger than or equal to) a second predefined size/shape. The second predefined size/shape can be an M×N block, or area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. When the chroma partition size is smaller than or equal to (or smaller than) the second predefined size/shape, the chroma partition can be different from the luma partition. It don't need to follow the luma split result. For example, the second predefined size can be 256 (in chroma sample precision, which is 1024 in luma sample precision in 4:2:0 chroma format). When the chroma CU size is larger than 256, it needs to follow the corresponding luma split type and split direction. When the chroma CU size is equal to or smaller than 256, the chroma split decision can be different the corresponding luma split type and split direction. This constraint can be also applied with above inferred split constraint. For example, the luma and chroma components are inferred split to 64×64 luma blocks and 32×32 chroma blocks (for example, in 4:2:0 chroma format). Under this 32×32 when the chroma block size is larger than 256, it needs to follow the corresponding luma split type and split direction. When the chroma block size is equal to or smaller than 256, the chroma split decision can be different from the corresponding luma split type and split direction.

In another embodiment, some exceptions can be allowed for the chroma split that one or more different split type and split direction can be applied if one or more conditions are satisfied. For example, if the second predefined size is 256 and the chroma blocks are inferred split to 32×32 blocks, some exceptions of chroma split are allowed. One exception is that, if the luma component use QT-split to split a luma 64×64 block to four luma 32×32 blocks, the chroma can use horizontal BT split to split a 32×32 chroma block to two chroma 32×16 blocks. The chroma 32×16 can be a leaf CU, or the chroma 32×16 block can be further split by using vertical BT split. After the vertical BT split, the size of 16×16 chroma block is equal to 256. It does not need to follow the luma split.

Another exception is that, if the luma component use a horizontal BT split followed by a vertical BT split to split a luma 64×64 block into four 32×32 luma blocks, the chroma can use QT-split to split a 32×32 chroma block to four chroma 16×16 blocks. After the QT-split, the size of 16×16 chroma block is equal to 256. It does not need to follow the luma split. In a more general way, the QT-split can be mapped to a horizontal BT split followed by a vertical BT split. For example, one can use the QT-split, and the other can use a horizontal BT split followed by a vertical BT split, and vice versa. In another more general way, for a chroma split, if each of the split nodes can cover and contain one or more complete luma CUs and the coding order of these luma CUs are continuous, the chroma split is valid (i.e., counted as following the luma split). The condition that chroma node “covers and contains” a complete luma CU as mentioned above means that if any part of the luma CU is covered by the chroma node, the complete luma CU is contained inside the chroma node. For example, chroma CU c in FIG. 7 covers and contains complete luma CUs D, E, F, G, H and I. Therefore, chroma CU c satisfies the condition for following the luma split. On the other hand, chroma CU b covers and contains complete luma CU C. However, chroma CU b covers and contains partial luma CU B. Therefore, chroma CU b satisfies the condition for following the luma split.

For luma and chroma CU syntaxes signalling, two methods can be applied. In the first method, the luma component and chroma component syntaxes are signalled in an interleaved fashion for a predefined size/shape. The predefined size/shape can be the same as the maximum transform unit size, an M×N block, or an area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64) or a derived block size, maximum TU size, a size related to maximum/minimum TU size, an adaptive shape block with an area equal to a predefined or a derived block area (e.g. block area equal to 4096 or the virtual pipeline data unit (VPDU) area/size). The luma component syntaxes of the predefined size/shape are signalled first, then the chroma component syntaxes of the predefined size/shape are signalled. In one example, the CTU is inferred to split to the predefined size/shape. For each block with the predefined size/shape, the luma component syntaxes are signalled before the chroma component syntaxes. In another example, the leaf CU size can be larger than the predefined size/shape. When the leaf CU size can be larger than the predefined size/shape, the luma and chroma share the same partition. The TU tiling syntaxes signalling is applied. The leaf CU can be divided into multiple TUs. For each TU, the luma coefficients are signalled before the chroma coefficients. In the second method, the luma component and chroma component syntaxes are signalled in an interleaved fashion when luma CU is not split or chroma CU is not split. For example, when chroma_split_end is true, the luma CU can be further split. At this level of CU, the luma CUs (including all the child CUs at this level) syntaxes are signalled first, then signal the chroma CU syntaxes. When the chroma_split_end is false and the luma CU stops split, the chroma CU can be further split. At this level of CU, the luma CU syntaxes are first signalled, then the chroma CUs (including all the child chroma CUs at this level) syntaxes are signalled. If the luma and chroma CU stop at the same level, the luma syntaxes are signalled before the chroma syntaxes.

In entropy coding, the context coded bin or bypass bin can be used to signal the chroma CU split syntaxes (e.g. the chroma_split_end or the chroma CU split syntax) when luma CU stops split. When using context coded bin, the neighbouring chroma CU information can be used for context modelling. The chroma_split_end can use the same context modelling or the same context modelling and the same context as the separated tree chroma split syntax. For example, when luma is doing QT split or current chroma CU needs to determine whether to use QT split or not, the chroma_split_end can reuse the context modelling or the context modelling and the context of the QT split or chroma QT split. For determining chroma BT or TT split, the BT/TT no-split flag context modelling or the context modelling and the context are used to determine the chroma_split_end flag value.

In one embodiment, when the luma CU undergoes QT split, the chroma QT split flag is signalled to determine whether the chroma CU follows the luma CU split. When the luma CU undergoes BT or TT split, the chroma no-split flag or the MTT split flag (e.g. mtt_split_cu_flag) is used to determine whether the chroma CU follows the luma CU split. If the chroma CU follows the luma CU split, the split type (e.g. QT/Hor. BT/Ver. BT/Hor. TT/Ver. TT) also follows the luma split type. The syntaxes of split direction and BT/TT selection (e.g. mtt_split_cu_vertical_flag and mtt_split_cu_binary_flag) can be skipped and inferred. In one example, when the luma CU undergoes BT or TT split, the chroma QT-split flag is not signalled. In this case, the chroma QT-split flag is inferred as false. When the luma block stops splitting, the chroma block split syntaxes (e.g. qt_split_cu_flag, mtt_split_cu_flag, mtt_split_cu_vertical_flag, and mtt_split_cu_binary_flag) can be signalled.

In the proposed method, the chroma split syntaxes (e.g. chroma qt_split_cu_flag, chroma mtt_split_cu_flag, chroma mtt_split_cu_vertical_flag, chroma mtt_split_cu_binary_flag, chroma_split_end, and/or chroma_follow_luma_split_flag) can be signalled before or after the luma split syntaxes. In one embodiment, the chroma_split_end or the chroma_follow_luma_split_flag is signalled before the luma split syntaxes. In another embodiment, the chroma_split_end or the chroma_follow_luma_split_flag is signalled after the luma split syntaxes. In another embodiment, the chroma qt_split_cu_flag, chroma mtt_split_cu_flag, chroma mtt_split_cu_vertical_flag, and/or chroma mtt_split_cu_binary_flag, chroma_split_end are signalled after luma split syntaxes. In another embodiment, the chroma_split_end or the chroma_follow_luma_split_flag is signalled after the luma split syntaxes and before the luma CU data syntaxes. In another embodiment, the chroma qt_split_cu_flag, chroma mtt_split_cu_flag, chroma mtt_split_cu_vertical_flag, and/or chroma mtt_split_cu_binary_flag, chroma_split_end are signalled after luma split syntaxes and before the luma CU data syntaxes. In another embodiment, the chroma split syntaxes (e.g. chroma qt_split_cu_flag, chroma mtt_split_cu_flag, chroma mtt_split_cu_vertical_flag, chroma mtt_split_cu_binary_flag, chroma_split_end, and/or chroma_follow_luma_split_flag) are signalled after the luma CU data syntaxes. In another embodiment, when luma CU is split, the chroma_split syntaxes (e.g. chroma qt_split_cu_flag, chroma mtt_split_cu_flag, chroma mtt_split_cu_vertical_flag, chroma mtt_split_cu_binary_flag, chroma_split_end, and/or chroma_follow_luma_split_flag) are signalled after the luma split syntaxes and before the luma CU data syntaxes. When the luma CU is not split, the chroma split syntaxes are signalled after the luma CU data syntaxes.

In one embodiment, the contexts of chroma CU split can be different from the luma CU split. An additional context set is used for chroma CU split. In another embodiment, the luma chroma can share the same context set for CU split syntaxes. In another embodiment, the separate context set are used for QT-split and/or MTT-split (e.g. qt_split_cu_flag and/or mtt_split_cu_flag) for the luma and chroma components, the share context set is used for the syntaxes of the split direction and BT/TT selection (e.g. mtt_split_cu_vertical_flag and mtt_split_cu_binary_flag).

In one embodiment, the concept of chroma split following luma split is applied, but the syntax signalling for chroma split is unchanged. Only the encoder follows the constraint. The syntax presentation is not subject to constraints. The encoder incorporates the constraint of chroma split following luma split to generate the syntax and bitstream. It is a bitstream conformance requirement that the chroma split type and split direction shall follow the methods as described above.

In another embodiment, the concept of chroma split following the luma split can be applied to LM mode constraints (for example, the LM syntax signalling or LM mode enabling). In this embodiment, there is no constraint (e.g. chroma split shall follow luma split) on the chroma split syntaxes. However, the LM is only enabled when the chroma split follows the luma split. The concept of the above proposed method can be applied. For example, for a chroma CU, from the CTU or a certain level, if the chroma splits follow the luma splits, the LM is enabled. The level can be predefined, derived, or signalled. In the case that the luma and chroma components use the same partition before the luma component stops splitting and the chroma component continues to split after the luma component stops splitting, the LM is enable. Otherwise, the LM is disabled. In another embodiment, in the case that the luma and chroma components use the same partition before the chroma component stops splitting and the luma component continues to split after the chroma component stops splitting, the LM is enable. Otherwise, the LM is disabled. For a chroma split node, if the chroma split doesn't follow the luma split, the LM cannot be applied to the leaf CUs within this node. In another example, for a chroma leaf CU, from the CTU or a certain level, if all its chroma splits follow the luma splits, the LM is enabled for this leaf CU. For a chroma node, if its corresponding luma partition stops and the chroma component and luma component have the same split before this node (e.g. the same split for all its parent nodes (or called all its ancestor nodes)), the LM is enabled for all leaf CUs within this node (or called all its descendant CUs following this node). Otherwise, the LM cannot be applied. For a chroma split node, if the chroma split doesn't follow the luma split, the LM cannot be applied to the leaf CUs within this node. In one example, if the LM cannot be applied, the LM mode syntax is not signalled and inferred as not selected. In another example, the LM mode syntax is still signalled, but, however, the LM cannot be selected. It is a bitstream conformance requirement that the LM mode cannot be selected in this condition. In still another example, the LM mode syntax is still signalled. However, if the chroma split doesn't follow the luma split for a chroma split node, the LM cannot be applied to the leaf CUs within this node and another non-LM chroma Intra mode is applied instead. Another non-LM chroma Intra mode can be planar, DC, vertical, horizontal, or vertical-diagonal chroma Intra mode.

In one embodiment, the LM mode constraints can be applied when the chroma component follows the luma split until the size/shape of chroma split node is larger than (or larger than or equal to) the second predefined size/shape. For a chroma CU, if it follows the luma split from its ancestor node(s) until a third size/shape, and the third size/shape is equal to or smaller than the second size/shape, the LM is enabled. For example, the second predefined size can be 256 in chroma sample resolution. For a chroma CU, if it is a leaf CU (or called a descendant CU) following the luma split from a CTU or a certain level, the LM is enabled. If the chroma CU size is smaller than 256 and it follows the luma split until a node, and the size of the node is equal to or smaller than 256, the LM is enabled. Also, similar to the methods above, some exceptions can be allowed for the LM mode if one or more conditions are satisfied. For example, if the second predefined size is 256 and the chroma blocks are inferred split to 32×32 blocks, some exceptions of chroma LM are allowed (in other words, chroma LM are allowed to be enabled under some exceptions). According to one exception, if the luma component uses QT-split to split a luma 64×64 block to four luma 32×32 blocks, and the chroma uses horizontal BT split to split a 32×32 chroma block to two chroma 32×16 blocks and the 32×16 block is a leaf CU, the LM mode is enabled for this 32×16 chroma leaf CU. According to another exception, the chroma 32×16 block can be further split by using vertical BT split. After the vertical BT split, the size of 16×16 chroma block is equal to 256. For this 16×16 chroma block or its children CUs (or called its descendant CUs), the LM is enabled. According to another exception, if the luma component uses a horizontal BT split followed by a vertical BT split to split a luma 64×64 block into four 32×32 luma blocks, the chroma component uses QT-split to split a 32×32 chroma block to four chroma 16×16 blocks. After the QTsplit, the size of 16×16 chroma block is equal to 256. For this 16×16 chroma block or its children CUs (or called its descendant CUs), the LM is enabled. In a more general way, the QTsplit can be mapped to a horizontal BT split followed by a vertical BT split. For example, one can use the QTsplit, the other can use a horizontal BT split followed by a vertical BT split. This case is still counted as “following the split”, and vice versa.

In another more general way, for a chroma split, if the split nodes can cover and contain one or more complete luma CUs and the coding order of these luma CUs is continuous, the chroma split is valid (e.g. counted as following the luma split). The condition that chroma node “covers and contains” a complete luma CU as mentioned above means that if any part of the luma CU is covered by the chroma node, the complete luma CU is contained inside the chroma node. For example, chroma CU c in FIG. 7 covers and contains complete luma CUs D, E, F, G, H and I. Therefore, chroma CU c satisfies the condition for following the luma split. On the other hand, chroma CU b covers and contains complete luma CU C. However, chroma CU b covers and contains partial luma CU B.

Therefore, chroma CU b satisfies the condition for following the luma split. In other words, for a chroma leaf CU, from the CTU or a certain level, if all its chroma splits are valid, the LM is enabled for this leaf CU. For a chroma node, if its corresponding luma partition stops and its chroma splits are valid before this node, the LM is enable for all leaf CU within this node. In another example, for a chroma leaf CU, if its size is equal to or larger than the second predefined size/shape and all its splits are valid, the LM mode can be applied for this leaf CU. For a chroma node, if its corresponding luma partition stops and its chroma splits are valid before this node, the LM is enable for all leaf CU within this node. For a chroma split node with size equal to or smaller than the second predefined size/shape, if all its splits are valid, all the leaf CUs within this node can use the LM mode. In another example, for a chroma split node, if its size is larger than or equal to the second predefined size/shape and not all its split are valid, the LM cannot be enabled for all the leaf CU within this node. In another example, for a chroma split node, if the split is not valid and the size of the child split node is equal to or larger than (or just larger than) the second predefined size/shape, the LM mode cannot be enabled for all the leaf CU within the child split node. In another example, the LM mode syntax is still signalled, however, the LM cannot be selected if the split check of a chroma node is not valid. It is a bitstream conformance requirements that the LM mode cannot be selected in this condition. In another example, the LM mode syntax still can be signalled even if the split check of a chroma node is not valid to apply LM mode. But if the split check of a chroma node is not valid and the LM mode syntax is signalled, then another non-LM chroma Intra mode is applied instead. Another non-LM chroma Intra mode can be planar, DC, vertical, horizontal, or vertical-diagonal chroma Intra mode.

Note that, the proposed LM mode constraint method can also be applied to other cross-colour component tools. The tools that need the information of different component (e.g. chroma needs luma information) can apply the similar constraints. For example, if the tools that need the reconstruction samples of the other colour component (e.g. chroma needs the luma reconstruction samples), the proposed constraints can be applied.

In another embodiment, the similar concept of using syntax of chroma_split_end can be implemented by using another syntax design. For example, the luma coding tree is first encoded or decoded. When coding the luma leaf CU or after coding the luma leaf CU, a syntax is encoded/parsed to indicate the chroma leaf CU size. The syntax can be related to the partition depth (e.g. QT-depth, BT-depth, TT-depth, CT-depth, and/or total-depth). For example, such syntax can be called chroma_depth_above. If the chroma_depth_above is equal to 0, it means the chroma CU and luma CU use the same partition and have the same CU size (in luma component unit). In the case of 4:2:0 format, the actual chroma CU size is ¼ of the actual luma CU size. If the chroma_depth_above is larger than 0 (e.g. equal to K), it means the chroma CU partition is stopped at K-depth above. For example, if a luma leaf CU has QT-depth equal to 3 and CT-depth equal to 0, the maximum CTU size is 128 and the size of this luma leaf CU is 16×16. If the chroma_depth_above is equal to 1, it means the chroma CU size is 32×32. The QT-depth of the chroma CU is 2. In this example, the chroma_depth_above syntax is not signalled for the following luma leaf CU if the chroma_depth_above is signalled in this 32×32 area. For example, for the following 3 luma QT CUs with size equal to 16×16 (no need to be a leaf CU), the chroma_depth_above syntax is not signalled. The first luma leaf CU in the next 32×32 area, the chroma_depth_above syntax is signalled. The chroma CU syntax can be signalled after the first luma leaf CU or after the 32×32 area. The 32×32 area is used as an example based on the chroma_depth_above value described in the above example.

In another embodiment, the chroma LM mode is taken into consideration. Since the LM mode is a powerful coding tool to compress the chroma component data, the LM mode syntax short cut can be applied with the method disclosed above. For example, the LM syntax can be applied when the chroma_split_end is equal to 1 or when the chroma_depth_above is signalled. In one embodiment, the LM mode has chance to be enabled (or also called as allowed) when the chroma CU size (in luma sample size) is larger than or equal to the corresponding luma CU size. In another embodiment, the LM mode has chance to be enabled (or also called as allowed) when the chroma_split_end equal to 1 or when the chroma_depth_above is signalled or when the luma and chroma CU stop at the same level (e.g. luma and chroma share the same partition tree in the above level and luma is not split and chroma is not split at the same level). If the luma partition is stopped and the chroma is further split, the LM mode is disabled.

In another embodiment, the adaptive chroma mode syntax is proposed. The chroma mode syntax can be the Intra prediction mode related syntax, motion information or transform/residual related syntax. The chroma syntax order/codeword can be different when the chroma CU size or depth is smaller than, equal to, or larger than the corresponding luma CU size or depth. For example, if the chroma CU size is smaller than the corresponding luma CU size or if the chroma CU depth is larger than the corresponding luma CU depth, the DM mode syntax or LM mode syntax is moved backward. For example, we can move the DM mode or LM mode candidate backward in most probable mode list or move the DM mode syntax or LM mode syntax after the normal syntax position.

In another embodiment, one can specify the maximum partition depth or the minimum block size for which the unified partitioning for luma and chroma is allowed. The maximum partition depth or the minimum block size can be signalled in high-level syntax at the sequence/picture/slice/tile group/tile-level such as SPS/PPS/VPS/slice-header/tile group header/tile header. Once the partition depth exceeds the maximum depth or the block size becomes smaller than the minimum size, only one of the luma and chroma components is allowed to be further split. In another embodiment, if the partition depth is smaller than the maximum partition depth or the block size is larger than the minimum block size, the unified partitioning for luma and chroma is applied. Once the partition depth exceeds the maximum depth or the block size becomes smaller than the minimum size, the separate luma/chroma coding tree is applied.

In another embodiment, the luma CU structure and chroma CU structure should have some correlation in terms of coding unit splitting. To reduce the encoding runtime and improve the coding efficiency, the chroma CU split should follow the luma CU split, but the chroma CU split can be early terminated or can be further split. For example, if the corresponding luma CU is using QT split/BT vertical split/BT horizontal split, the chroma CU has two options. One is to follow the luma split type, and the other one is not to split. If the corresponding luma CU is not split, the chroma CU can be further split. In another embodiment, the chroma CU cannot be split. The early termination flag or following split flag can be inferred under some constraints. For example, the chroma CU split always follows luma CU split in the first K layers. In another example, when CU size is larger than M×N, the chroma CU split always follows the luma CU split. In another example, the chroma QT split always follows the luma QT split. The early stop flag is only for the BT split.

The constraint can be adaptive. For example, the K, M, and N can be derived by the decoded CUs or the neighbouring CUs.

In another embodiment, the proposed guided CU split is only applied when the chroma CU and luma CU have the same QT depth, which means the chroma BT split follows the luma BT split if their QT leaf-CUs are the same size.

In another example, the chroma QT split always follows luma QT split (i.e., sharing the same QT split), but there is no constraint for the BT split.

In one embodiment, the CTU can be split into child CUs. When the CU size/shape is larger than a predefined size/shape, the shared tree partition is applied. If the CU size/shape is equal to or smaller than the predefined size/shape, the separated tree partition is applied. The predefined size/shape can be the same as the maximum transform unit size, or can be an M×N block or an area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64 or 32×32), a derived block size, the maximum TU size, a size related to maximum/minimum TU size, or an adaptive shape block with area equal to a predefined or a derived block area (e.g. block area equal to 4096, or equal to the virtual pipeline data unit (VPDU) area/size). In one example, the CTU is inferred to split into multiple second blocks with a predefined size/shape, then the proposed method mentioned above is applied to the multiple second blocks with the predefined size/shape. The second predefined size/shape can be the same as the maximum transform unit size, or can be an M×N block or area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64 or 32×32), a derived block size, the maximum TU size, a size related to maximum/minimum TU size, or an adaptive shape block with area equal to a predefined or a derived block area (e.g. block area equal to 4096, or equal to the virtual pipeline data unit (VPDU) area/size). For example, the CTU can be inferred to split or not inferred to split into multiple 64×64 blocks first. For each 64×64 block, if the CU size is larger than 32×32, the shared tree partitioning is applied. If the CU size is equal to or smaller than 32×32, the separated tree partitioning is applied.

In another embodiment, the luma CU structure and chroma CU structure should have some correlation in terms of coding unit splitting. The chroma CU split should follow the luma CU split, but the chroma CU split can be early terminated or can be further split. For example, if the corresponding luma CU is using QT split/BT vertical split/BT horizontal split, the chroma CU has two options. One is to follow the luma split type, and the other one is not to split. In still another example, a CU size/shape threshold is predefined. The CU size/shape threshold can be the same as the maximum transform unit size, an M×N block, or area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64 or 32×32) or a derived block size, the maximum TU size, a size related to maximum/minimum TU size, an adaptive shape block with area equal to a predefined or a derived block area (e.g. block area equal to 4096, or equal to or smaller than the virtual pipeline data unit (VPDU) area/size), or a maximum allowed BT/TT split size. If the chroma CU size is larger than the CU size/shape threshold (e.g., 32×32 or 16×16), the chroma CU has two options. One is to follow the luma split type, and the other one is not to split. Otherwise, if the chroma CU size is smaller than or equal to the CU size/shape threshold (e.g., 32×32 or 16×16), then the separated tree partitioning is applied. In still another example, if cross-colour component prediction modes (e.g., LM mode) is disallowed for the current picture/brick/tile/slice/CTU, then the luma CU structure and chroma CU structure can have different correlation in terms of coding unit splitting. In still another example, if the corresponding luma CU is not split, then separated tree partitioning is applied.

In another embodiment, if a cross-colour component prediction mode (e.g., LM mode) is allowed for the current picture/brick/tile/slice/CTU, then the luma CU structure and chroma CU structure should have some correlation in terms of coding unit splitting for enabling the cross-colour component prediction mode. The chroma CU split should follow the luma CU split, but the chroma CU split can be early terminated or can be further split. For example, if the corresponding luma CU is using QT split/BT vertical split/BT horizontal split, the chroma CU has two options. One is to follow the luma split type, and the other one is not to split. In still another example, a CU size/shape threshold is predefined. The CU size/shape threshold can be the same as the maximum transform unit size, an M×N block, or area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64 or 32×32) or a derived block size, the maximum TU size, a size related to maximum/minimum TU size, an adaptive shape block with area equal to a predefined or a derived block area (e.g. block area equal to 4096, or equal to or smaller than the virtual pipeline data unit (VPDU) area/size), or a maximum allowed BT/TT split size. If the chroma CU size is larger than the CU size/shape threshold (e.g., 32×32 or 16×16), the chroma CU has two options. One is to follow the luma split type, and the other one is not to split. Otherwise, if the chroma CU size is smaller than or equal to the CU size/shape threshold (e.g., 32×32 or 16×16), then the separated tree partitioning is applied. In still another example, if the corresponding luma CU is not split, then separated tree partitioning is applied.

In still another embodiment, if a cross-colour component prediction mode (e.g., LM mode) is allowed for the current picture/brick/tile/slice/CTU and separate partitioning for luma and chroma is also enabled, the luma CU structure and chroma CU structure should have some correlation in terms of coding unit splitting. Therefore, a pipeline processing threshold is predefined to limit the maximum allowed BT/TT split size for both luma and chroma CU structure. When the current chroma CU size and the corresponding luma CU size is larger than the pipeline processing threshold, then only no split and QT split can be applied. For example, if the pipeline processing threshold is 32×32, then the maximum allowed BT/TT split size for luma is equal to or smaller than 32×32, and the maximum allowed BT/TT split size for chroma is equal to or smaller than 32×32 (in luma sample unit size). Therefore, when the current chroma CU size is 64×64 (in luma sample unit size), only no split and QT split are allowed. Similarly, when the corresponding luma CU size is 64×64, only no split and QT split are allowed.

In still another embodiment, if a cross-colour component prediction mode (e.g., LM mode) is allowed for the current picture/brick/tile/slice/CTU and separate partitioning for luma and chroma is also enabled, the luma CU structure and chroma CU structure should have some correlation in terms of coding unit splitting. For example, to reduce the luma-chroma latency issue, from 64×64 to 32×32 luma samples. For each 64×64 luma node, only no split or QT split can be applied. For the corresponding 32×32 chroma node of the 64×64 luma node, only no split, QT split, or horizontal BT split can be applied. Also, if horizontal BT split is applied to 32×32 chroma node, then no split is applied to the further 32×16 chroma nodes (i.e., coding blocks).

In still another embodiment, if a cross-colour component prediction mode (e.g., LM mode) is allowed for the current picture/brick/tile/slice/CTU and separate partitioning for luma and chroma is also enabled, the luma CU structure and chroma CU structure should have some correlation in terms of coding unit splitting. For example, to reduce the luma-chroma latency issue, from 64×64 to 32×32 luma samples. For each 64×64 luma node, only no split or QT split can be applied. For the corresponding 32×32 chroma node of each 64×64 luma node, only no split, QT split, or horizontal BT split can be applied. Also, if horizontal BT split is applied to 32×32 chroma node, then no split or vertical BT split is applied to the further 32×16 chroma nodes.

In still another embodiment, a CTU is inferred split or not inferred split into several 64×64 blocks first, and each 64×64 luma split nodes is allowed using no split or QT split. If a cross-colour component prediction mode (e.g., LM mode) is allowed for the current picture/brick/tile/slice/CTU and separate partitioning for luma and chroma is also enabled, the split type of parent chroma node of each chroma leaf node and the associated split type of the parent luma node is checked to allow the LM mode for the current chroma leaf node. For example, if no split is applied to a 64×64 luma node, then LM mode can be used for the current chroma leaf node. If only QT split is applied to a 64×64 luma node, then LM mode can be used for the current chroma leaf node when QT split is also applied to the 32×32 chroma parent node. If only QT split is applied to a 64×64 luma node, then LM mode can be used for the current chroma 32×16 leaf node when horizontal split is also applied to the 32×32 chroma parent node. If only QT split is applied to a 64×64 luma node, then LM mode can be used for the current chroma 16×16 leaf node when horizontal split is applied to the 32×32 chroma parent node and then vertical split is applied to the next 32×16 chroma parent node. In another example, the LM mode syntax is still signalled, however, the LM cannot be selected if the split check of a chroma node is not valid. It is a bitstream conformance requirements that the LM mode cannot be selected in this condition. In another example, the LM mode syntax still can be signalled even if the split check of a chroma node is not valid to apply LM mode. However, if the split check of a chroma node is not valid and the LM mode syntax is signalled, then another non-LM chroma Intra mode is applied instead. Another non-LM chroma Intra mode can be planar, DC, vertical, horizontal, or vertical-diagonal chroma Intra mode. The parent node is also referred as an ancestor node. A parent node of the parent node is also referred as an ancestor node in this disclosure. Furthermore, the ancestor node of the parent node is also referred as an ancestor node.

In still another embodiment, a CTU is inferred split or not inferred split into several 64×64 blocks first, and each 64×64 luma split nodes is allowed using no split or QT split. If a cross-colour component prediction mode (e.g., LM mode) is allowed for the current picture/brick/tile/slice/CTU and separate partitioning for luma and chroma is also enabled, the split type of the parent chroma node of each chroma leaf node is checked to allow the LM mode for the current chroma leaf node. For example, the LM mode can be used for the current chroma leaf node if the current chroma leaf node size is 32×32. When QT split is applied to the 32×32 chroma parent node, then LM mode can be used for the current chroma leaf node. When horizontal split is applied to the 32×32 chroma parent node, and the current chroma leaf node size is 32×16, then LM mode can be used for the current chroma leaf node. When the horizontal split is applied to the 32×32 chroma parent node and then vertical split is applied to the next 32×16 chroma parent node, then LM mode can be used for the current chroma leaf node. In another example, the LM mode syntax is still signalled; however, the LM cannot be selected if the split check of a chroma node is not valid. It is a bitstream conformance requirement that the LM mode cannot be selected in this condition. In another example, the LM mode syntax still can be signalled even if the split check of a chroma node is not valid to apply LM mode. However, if the split check of a chroma node is not valid and the LM mode syntax is signalled, then another non-LM chroma Intra mode is applied instead. Another non-LM chroma Intra mode can be planar, DC, vertical, horizontal, or vertical-diagonal chroma Intra mode.

The CTU can be split into child CUs, and pipeline processing grids are also predefined for alleviating the latency issue caused by cross-colour component prediction. The predefined pipeline processing grids can be the same as the maximum transform unit size, an M×N block, or area equal to 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, or 8192. The M×N block can be a predefined (e.g. 64×64 or 32×32) or a derived block size, equal to maximum TU size, a size related to maximum/minimum TU size, an adaptive shape block with area equal to a predefined or a derived block area (e.g. block area equal to 4096, or equal to or smaller than the virtual pipeline data unit (VPDU) area/size), or a maximum allowed BT/TT split size. The CTU can be inferred split or not inferred split into multiple M×M blocks first for both luma and chroma components. For each M×M block, the separated tree partitioning can be applied; however, implicit TU split is applied to chroma CU structure according to pipeline processing grids. Besides, for a chroma leaf CU coded in Intra mode, the top-right and below-left neighbouring reconstructed reference samples across predefined pipeline processing grids are set as “not available for Intra prediction”, unless the neighbouring reconstructed reference samples are from neighbouring M×M blocks.

In one embodiment, if a CTU is inferred split or not inferred split into several 64×64 blocks first, and the luma CU is allowed using no split, QT split, BT vertical split or BT horizontal split, the pipeline processing grids for chroma are set to 16×16 grids. As shown in the right side of FIG. 8, implicit TU split according to 16×16 pipeline processing grids (i.e., marked in dash lines) is applied to chroma 32×32 block 820 only when chroma CU is further split. For example, if the resulting chroma 32×32 block split is firstly split by QT split into 4 final CUs (i.e., 16×16 CUs, 821), then each final CU is not further implicit TU split due to the CU size is smaller than or equal to 16×16 grid. In another example, if the resulting chroma 32×32 block is firstly split by BT vertical split into 2 final CUs (i.e., 16×32 CUs), then each final CU is further implicitly split into two 16×16 TUs. In still another example, if the resulting chroma 32×32 block is firstly split by BT horizontal split into 2 final CUs (i.e., 32×16 CUs, 822), then each final CU is further implicitly split into two 16×16 TUs, 824. In still another example, if the resulting chroma 32×32 block split is firstly split by TT vertical split into 3 final CUs 825 (i.e., two 8×32 CUs, one 16×32 CU), then the 8×32 CUs are implicitly split into two 8×16 TUs. The center 16×32 CU is implicitly split into four 8×16 TUs. For still another example, if the resulting chroma 32×32 block is firstly split by TT horizontal split into 3 final CUs 823 (i.e., two 32×8 CUs, one 32×16 CU), then the 32×8 CUs are implicitly split into two 16×8 TUs, and the center 32×16 CU is implicitly split into four 16×8 TUs. Note that, the example shows here is only for the 32×32 chroma block, where each resulting sub-CUs can be further split into smaller sub-CUs. However, if a sub-CU is not entirely inside a 16×16 pipeline processing grid, then the sub-CU is further implicitly split by the 16×16 pipeline processing grids as the examples for 32×32 chroma block given in FIG. 8. The allowed luma CU split is shown on the left side of FIG. 8 for non-split 810, horizontal binary split 812, QT split 814 and vertical binary split 816.

Another example according to the present invention is shown in top illustration 910 of FIG. 9, where a chroma leaf node marked as “A” is coded in Intra mode. In FIG. 9, the top-right and below-left neighbouring reconstructed reference samples across 16×16 pipeline processing grids (i.e., marked by slash lines) are set as “not available for Intra prediction”. For the example shown in bottom illustration 920 of FIG. 9, a chroma leaf node marked as “B” is coded in Intra mode. While the below-left neighbouring reconstructed reference samples are across 16×16 pipeline processing grids (i.e., marked by slash lines), these neighbouring reconstructed reference samples are still available for Intra prediction for “B” since these samples are from neighbouring 32×32 blocks. Besides, if the corresponding 64×64 luma CU is not split, then no implicit TU split is applied to chroma CU structure.

In one embodiment, if a CTU is inferred split or not inferred split into multiple 64×64 blocks first, and the luma CU is allowed using no split 1010, QT split 1013, BT vertical split 1014, BT horizontal split 1011, TT vertical split 1015, and TT horizontal split 1012, the pipeline processing grids for chroma are set to 8×8 grids. As shown in the right side of FIG. 10, implicit TU split according to 8×8 pipeline processing grids (i.e., marked in dash lines) is applied to chroma 32×32 block only when chroma CU is further split. For example, if the resulting chroma 32×32 block split 1020 is firstly split by QT split into 4 final CUs (i.e., 16×16 CUs, 1021), then each final CU is further split into four 8×8 TUs. In another example, if the resulting chroma 32×32 block split is firstly split by BT vertical split into 2 final CUs (i.e., 16×32 CUs, 1024), then each final CU is further split into eight 8×8 TUs. In still another example, if the resulting chroma 32×32 block split is firstly split by BT horizontal split into 2 final CUs (i.e., 32×16 CUs, 1022), then each final CU is further split into eight 8×8 TUs.

In still another example, if the resulting chroma 32×32 block split is firstly split by TT vertical split into 3 final CUs 1025 (i.e., two 8×32 CUs, one 16×32 CU), then the 8×32 CUs is implicitly split into four 8×8 TUs, and the center 16×32 CU is implicitly split into eight 8×8 TUs. For still another example, if the resulting chroma 32×32 block split is firstly split by TT horizontal split into 3 final CUs 1023 (i.e., two 32×8 CUs, one 32×16 CU), then the 32×8 CUs are implicitly split into four 8×8 TUs, and the center 32×16 CU is implicitly split into eight 8×8 TUs. Note that, the example shows here is only for the 32×32 chroma block, where each resulting sub-CUs can be further split into smaller sub-CUs. However, if a sub-CU is not entirely inside an 8×8 pipeline processing grid, then the sub-CU is further implicitly split by the 8×8 pipeline processing grids as the examples for 32×32 chroma block given in FIG. 10. Besides, as shown in FIG. 9, a chroma leaf node marked as “A” is coded in Intra mode, the top-right and below-left neighbouring reconstructed reference samples across 8×8 pipeline processing grids (i.e., marked by slash lines) are set as “not available for intra prediction”. For the example shown in FIG. 9, a chroma leaf node marked as “B” is coded in Intra mode, While the below-left neighbouring reconstructed reference samples are across 8×8 pipeline processing grids (i.e., marked by slash lines), these neighbouring reconstructed reference samples are still available for Intra prediction for “B” due to these samples are from neighbouring 32×32 blocks. Besides, if the corresponding 64×64 luma CU is not split, then no implicit TU split is applied to chroma CU structure.

In still another embodiment, if a CTU is inferred split or not inferred split into multiple 64×64 blocks first, and the luma CU is allowed using no split/QT split, the pipeline processing grids for chroma are set to 16×16 grids. As shown in the right side of FIG. 11, implicit TU split according to 16×16 pipeline processing grids (i.e., marked in dash lines) is applied to chroma 32×32 block. For an example, if a chroma 32×32 node 1120 is firstly split by QT split into four chroma 16×16 nodes 1121, then implicit TU split is not applied. In another example, if a chroma 32×32 node is firstly split by horizontal BT split into two chroma 32×16 leaf nodes (i.e., CBs, 1122), then implicit TU split is also not applied. In another example, if a chroma 32×32 node is firstly split by horizontal BT split into two chroma 32×16 nodes, and a 32×16 chroma node is further BT/TT split, then the implicit TU split at 16×16 grids is applied to the 32×16 node 1123. In another example, if a chroma 32×32 node is firstly split by vertical BT split into two chroma 16×32 nodes, then the implicit TU split at 16×16 grids is applied to the 16×32 chroma node 1124. The allowed luma CU split is shown on the left side of FIG. 11 for non-split 1110 and QT split 1112. Besides, as shown in FIG. 9, a chroma leaf node marked by “A” is coded in Intra mode and the implicit TU split is also applied to parent node, the top-right and below-left neighbouring reconstructed reference samples across 16×16 pipeline processing grids (i.e., marked by slash lines) are set as “not available for Intra prediction”.

For the example shown in FIG. 9, a chroma leaf node marked as “B” is coded in Intra mode and the implicit TU split is also applied to the parent node, while the below-left neighbouring reconstructed reference samples are across 16×16 pipeline processing grids (i.e., marked by slash lines), these neighbouring reconstructed reference samples are still available for Intra prediction for “B” since these samples are from neighbouring 32×32 blocks. In another example, if the implicit TU split is applied to the parent node, but the horizontal BT split is applied to a 32×32 parent node, then these neighbouring reconstructed reference samples are still available for Intra prediction of the current chroma leaf node. In another example, if the QT split is applied to 32×32 parent node, then these neighbouring reconstructed reference samples are still available for Intra prediction of the current chroma leaf node. Besides, if the corresponding 64×64 luma CU is not split, then no implicit TU split is applied to chroma CU structure.

Any of the foregoing proposed methods can be implemented in encoders and/or decoders. For example, any of the proposed methods can be implemented in an entropy encoding module, a prediction generator, or a partition control module at an encoder side. Any of the proposed methods can also be implemented in an entropy parser module, a prediction generator or a partition control module at a decoder side. Alternatively, any of the proposed methods can be implemented as a circuit coupled to the entropy encoding module, a predictor generator, or the partition control module at the encoder side. Any of the proposed methods can be implemented as a circuit coupled to the entropy parser module, a predictor generator, or the partition control module of the decoder, so as to provide the information needed by the entropy parser module or the partition control module.

FIG. 12 illustrates a flowchart of an exemplary coding system according to an embodiment of the present invention, where whether to allow the LM mode depends on splitting at the parent nodes. The steps shown in the flowchart, as well as other following flowcharts in this disclosure, may be implemented as program codes executable on one or more processors (e.g., one or more CPUs) at the encoder side and/or the decoder side. The steps shown in the flowchart may also be implemented based hardware such as one or more electronic devices or processors arranged to perform the steps in the flowchart. According to this method, input data associated with a current data unit in a current picture are received in step 1210, where the input data associated with the current data unit comprise a luma component and a chroma component. If separate partition trees are allowed for the luma block and the chroma block, the luma block is partitioned into one or more luma leaf blocks using a luma partition tree and the chroma block is partitioned into one or more chroma leaf blocks using a chroma partition tree in step 1220. If a cross-colour component prediction mode is allowed, whether to enable an LM (Linear Model) mode for a target chroma leaf block is determined based on a first split type applied to an ancestor chroma node of the target chroma leaf block and a second split type applied to a corresponding ancestor luma node in step 1230. The target chroma leaf block is encoded or decoded using the LM mode if the LM mode is enabled for the target chroma leaf block in step 1240.

FIG. 13 illustrates a flowchart of an exemplary coding system according to an embodiment of the present invention, where exception conditions are determined to allow the LM mode when the luma and chroma use different splitting. According to this method, input data associated with a current data unit in a current picture are received in step 1310, where the input data associated with the current data unit comprise a luma component and a chroma component. If separate partition trees are allowed for the luma block and the chroma block, the luma block is partitioned into one or more luma leaf blocks using a luma partition tree and the chroma block is partitioned into one or more chroma leaf blocks using a chroma partition tree in step 1320. Determine whether one or more exception conditions to allow an LM (Linear Mode) for a target chroma leaf block are satisfied when the chroma partition tree uses a different split type, a different partition direction, or both from the luma partition tree in step 1330. The target chroma leaf block is encoded or decoded using the LM mode in step 1340 if chroma partition tree is different from the luma partition tree, but at least one of the one or more exception conditions is satisfied.

The flowcharts shown are intended to illustrate an example of video coding according to the present invention. A person skilled in the art may modify each step, re-arranges the steps, split a step, or combine steps to practice the present invention without departing from the spirit of the present invention. In the disclosure, specific syntax and semantics have been used to illustrate examples to implement embodiments of the present invention. A skilled person may practice the present invention by substituting the syntax and semantics with equivalent syntax and semantics without departing from the spirit of the present invention.

The above description is presented to enable a person of ordinary skill in the art to practice the present invention as provided in the context of a particular application and its requirement. Various modifications to the described embodiments will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. In the above detailed description, various specific details are illustrated in order to provide a thorough understanding of the present invention. Nevertheless, it will be understood by those skilled in the art that the present invention may be practiced.

Embodiment of the present invention as described above may be implemented in various hardware, software codes, or a combination of both. For example, an embodiment of the present invention can be one or more circuit circuits integrated into a video compression chip or program code integrated into video compression software to perform the processing described herein. An embodiment of the present invention may also be program code to be executed on a Digital Signal Processor (DSP) to perform the processing described herein. The invention may also involve a number of functions to be performed by a computer processor, a digital signal processor, a microprocessor, or field programmable gate array (FPGA). These processors can be configured to perform particular tasks according to the invention, by executing machine-readable software code or firmware code that defines the particular methods embodied by the invention. The software code or firmware code may be developed in different programming languages and different formats or styles. The software code may also be compiled for different target platforms. However, different code formats, styles and languages of software codes and other means of configuring code to perform the tasks in accordance with the invention will not depart from the spirit and scope of the invention.

The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described examples are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

1. A method of video coding used by a video coding system, the method comprising: receiving input data associated with a current block in a current picture, wherein the input data comprise pixel data to be encoded at an encoder side or compressed data at a decoder side, and wherein the current block comprises a luma block and a chroma block; if separate partition trees are allowed for the luma block and the chroma block, partitioning the luma block into one or more luma leaf blocks using a luma partition tree and partitioning the chroma block into one or more chroma leaf blocks using a chroma partition tree; if a cross-colour component prediction mode is allowed, determining whether to enable an LM (Linear Model) mode for a target chroma leaf block based on a first split type applied to an ancestor chroma node of the target chroma leaf block and a second split type applied to a corresponding ancestor luma node; and encoding or decoding the target chroma leaf block using the LM mode if the LM mode is enabled for the target chroma leaf block.
 2. The method of claim 1, wherein the corresponding ancestor luma node has a block size of 64×64 and the ancestor chroma node of the target chroma leaf block has a block size of 32×32 in a 4:2:0 colour format.
 3. The method of claim 1, wherein when the second split type corresponds to no split and the first split type corresponds to no split, QT (quadtree) split, HorBT (horizontal binary tree) split with no further split, or HorBT split followed by VerBT (vertical binary tree) split, the LM mode for the target chroma leaf block is enabled.
 4. The method of claim 1, wherein when the second split type corresponds to QT (quadtree) split and the first split type corresponds to no split, QT (quadtree) split, HorBT (horizontal binary tree) split with no further split, or HorBT split followed by VerBT (vertical binary tree) split, the LM mode for the target chroma leaf block is enabled.
 5. An apparatus of video encoding and decoding used by a video encoding system and video decoding system respectively, the apparatus comprising one or more electronic circuits or processors arrange to: receive input data associated with a current block in a current picture, wherein the input data associated with the current block comprise a luma component and a chroma component and the current block comprises a luma block and a chroma block; if separate partition trees are allowed for the luma block and the chroma block, partition the luma block into one or more luma leaf blocks using a luma partition tree and partitioning the chroma block into one or more chroma leaf blocks using a chroma partition tree; if a cross-colour component prediction mode is allowed, determine whether to enable an LM (Linear Model) mode for a target chroma leaf block based on a first split type applied to an ancestor chroma node of the target chroma leaf block and a second split type applied to a corresponding ancestor luma node; and encode or decode the target chroma leaf block using the LM mode if the LM mode is enabled for the target chroma leaf block.
 6. A method of video coding used by a video coding system, the method comprising: receiving input data associated with a current block in a current picture, wherein the input data comprise pixel data to be encoded at an encoder side or compressed data at a decoder side, and wherein the current block comprises a luma block and a chroma block; if separate partition trees are allowed for the luma block and the chroma block, partitioning the luma block into one or more luma leaf blocks using a luma partition tree and partitioning the chroma block into one or more chroma leaf blocks using a chroma partition tree; determining whether one or more exception conditions to allow an LM (Linear Mode) for a target chroma leaf block is satisfied when the chroma partition tree uses a different split type, a different partition direction, or both from the luma partition tree; and encoding or decoding the target chroma leaf block using the LM mode if chroma partition tree is different from the luma partition tree, but at least one of the one or more exception conditions is satisfied.
 7. The method of claim 6, wherein said one or more exception conditions comprise a case that the luma block being a luma 64×64 block is partitioned into four luma 32×32 blocks using a QT (quadtree) split and the chroma block being a chroma 32×32 block is partitioned into two chroma 32×16 blocks using a horizontal BT (binary tree) split with no further split or partitioned into four chroma 16×16 blocks using the horizontal BT split followed by a vertical BT split.
 8. The method of claim 6, wherein said one or more exception conditions comprise a case that the luma block being a luma 64×64 block is partitioned into four luma 32×32 blocks using a horizontal BT (binary tree) split followed by a vertical BT split and the chroma block being a chroma 32×32 block is partitioned into four chroma 16×16 blocks using a QT (quadtree) split.
 9. The method of claim 6, wherein said one or more exception conditions comprise a case that the chroma block is partitioned into one or more split chroma CUs (coding units) and each of said one or more split chroma CUs cover and contain one or more complete luma CUs resulted from partitioning the luma block using the luma partition tree, and wherein coding order of said one or more complete luma CUs are continuous.
 10. An apparatus of video encoding and decoding used by a video encoding system and video decoding system respectively, the apparatus comprising one or more electronic circuits or processors arrange to: receive input data associated with a current block in a current picture, wherein the input data comprise pixel data to be encoded at an encoder side or compressed data at a decoder side, and wherein the current block comprises a luma block and a chroma block; if separate partition trees are allowed for the luma block and the chroma block, partition the luma block into one or more luma leaf blocks using a luma partition tree and partitioning the chroma block into one or more chroma leaf blocks using a chroma partition tree; determine whether one or more exception conditions to allow an LM (Linear Mode) for a target chroma leaf block is satisfied when the chroma partition tree uses a different split type, a different partition direction, or both from the luma partition tree; and encode or decode the target chroma leaf block using the LM mode if chroma partition tree is different from the luma partition tree, but at least one of the one or more exception conditions is satisfied. 